Conventionally, most timers used in ovens only control the cooking time. A variety of these are available, such as, a timer driven by a coil spring, a timer driven by a compact-size synchronous motor, and one that uses an electric circuit incorporating an IC, etc. However, since any one of these timers merely controls the cooking time in accordance with the kinds of food being processed, the temperature in the oven chamber greatly differs between the pre-cooking cold condition that is present and the heated condition after using the oven once. As a result, if the user starts foodcooking without regard to the actual temperature in the oven chamber, and the cooking was done within the same period as was done before, the result will be a great difference between both cases such that the food will either be excessively cooked or conversely will be raw.
To eliminate such a disadvantage, normally, conventional ovens perform a preheating operation until a threshold temperature, proper for starting cooking, is present in the oven chamber. For example, if a temperature of 180.degree. C. is needed for cooking when the initial ambient temperature, normally, the preheating is performed until temperature eventually reaches 180.degree. C. from 20.degree. C. To advise the operator of the completed preheating operation, either a buzzer or an indicator lamp is used. If an oven uses a thermometer to indicate the internal temperature, it is not necessary to directly advise the operator of the completed preheating operation. If the operator starts cooking only after completing the preheating operation, since the internal temperature will remain almost constant during cooking, food can be uniformly cooked. However, since the preheating operation described above needs a means for sensing temperature in the oven chamber, either bi-metal or thermistor should be provided. This makes the mechanism complex. Such a large-size oven having a temperature-control circuit for holding the cooking temperature constant can also effectively perform the preheating operation using a temperature control circuit. This can be done by first setting a specific temperature needed for cooking and then by activating the heater without placing any food in the chamber. Due to functions of the temperature control circuit, temperature in the oven chamber gradually rises, and then the internal temperature is held constant at a specific level designated in advance. On the other hand, a small oven, particularly a oven toaster, is not provided with any temperature sensor for advising the operator of the completed preheating operation, thus no preheating can be performed. In particular, when preparing toast using an oven toaster, since bread is inserted into the baking chamber before the internal temperature fully rises, and, since it is usually baked for a maximum of 3 or 4 minutes, it is extremely difficult to distinguish between the preheating and the actual baking effect. When making toast using an oven toaster that cannot perform any preheating operation, the following situations usually take place. When more than two pieces of bread are consecutively baked for the same amount of time, and if the first piece of bread was properly baked after being placed in the oven chamber that is cold, the second bread will be excessively toasted after being placed in the already heated chamber. Conversely, if a specific toasting time is set so that the second piece of bread can be properly toasted, then the first piece of bread cannot be toasted well. In addition, if a certain period of time is wasted between the toasting of the second piece of bread and the toasting of the first piece of bread, the situation will involve further complexity. This is because, when toasting the second piece of bread, the initial temperature for the second piece of bread is somewhere between the initial cold temperature, and the final cooking temperature of the first piece of bread. As is clear from these facts, any of the conventional oven toasters have a disadvantage in that a great difference occurs in the completion of toast and other foods depending on the the initial temperature inside the oven chamber when the cooking times are set equal to each other. To compensate for this, it has been a compulsory practice to set time very precisely in each cooking operation according to the temperature in the oven chamber, and yet, since the time-setting operation has to be done totally based on the operator's own experience and instinct, this has long been a difficult problem to solve. As time passes by, temperature in the oven chamber is subject to change. This change is delicately variable according to the condition of use. For example, the variable temperature inside the oven chamber is simulated by using an electric circuit. FIG. 1 shows a simplified block diagram of an electric equivalent circuit, in which the power W denotes the calorific unit called W, which is generated by the heater in one second. Resistors Rh and Ro are the thermal resistors connected to the heater and the oven chamber and also to the internal and external parts of the oven chamber, respectively, whose thermal resistance value is denoted in terms of .degree.C./W. Co denotes the thermal capacitor provided in the oven chamber, using a unit of sec./.degree.C. References Tth, To, and Ta respectively show the heater temperature, temperature in the oven chamber, and ambient temperature, which are denoted in terms of .degree.C. Temperatures in respective parts of the electric equivalent circuit correspond to the voltage at the contact points. Cx shown in parallel with Co denotes the thermal capacitance of food and this value is variable according to the kind of food. Referring now to FIG. 1, such a process is denoted by a simplified circuit diagram shown below, in which, after activating the heater at time T=0, temperature To in the oven chamber gradually rises from a specific level that is equal to ambient temperature. ##EQU1## where "exp" denotes the index function. The maximum temperature Tmax in the oven chamber that has been reached after a heating operation is denoted by the following equation, where 0 denotes the member of the index function. ##EQU2## Actually, when Ta=20.degree. C., the value Tmax will reach 320.degree. C. in the oven chamber. Assuming that the heater generates 1,000 W of the calorific value, the thermal resistance value is calculated to be ##EQU3## FIG. 2 shows the graphic summary of the process denoted by the equation (1). Temperature To in the oven chamber is equal to the ambient temperature Ta=20.degree. C. at the moment when t=0, while the internal temperature of the oven chamber gradually rises to Tmax=320.degree. C. by drawing an index curve. In FIG. 2, when temperature To in the oven chamber reaches the threshold temperature Tth above 100.degree. C., food can be cooked quite sufficiently. The threshold temperature level is different according to the kind of food. However, it is about 110.degree. C. when toasting bread. The preheating period is equal to such a period in which the initial temperature in the oven chamber rises from the initial temperature Ta to the threshold temperature Tth. In the electric equivalent circuit shown in FIG. 1, the thermal capacitor is substituted by the capacitance of capacitors, whereas the thermal resistance is also substituted by electric resistors, and thus, it is possible to almost realize the actual preheating operation of an oven by effectively operating an electric equivalent circuit.
FIG. 3 shows an example of a conventional oven preheating timer, in which a portion surrounded by broken line ( ) denotes a preheating timer. Reference PS indicates a DC power circuit. In FIG. 3, a charge circuit is formed for capacitor C by a specific time constant determined by resistor RA and capacitor C, whereas a discharge circuit is formed for capacitor C by a specific discharge time constant determined by resistor RB and capacitor C. Capacitor C corresponds to the thermal capacitor of the oven chamber. Relay 1 switches for charging and discharging capacitor C, which is then charged while the heater is still ON. A zener diode ZD connected to resistor RB in parallel controls the voltage during discharge. Relay 2 feeds the power to the heater 2. Relays 1 and 2 are simultaneously driven by the main timer MT. The main timer MT starts its counting operation when the output of the logic circuit 3 goes Low and keeps relays 1 and 2 activated while the output of the logic circuit 3 remains High. The start switch SW is pressed ON when starting a cooking operation. When this switch is pressed ON, since capacitor C has such a voltage higher than the input threshold voltage of the logic circuit 3, the voltage from the logic circuit 3 goes High, thus allowing the main timer MT to hold relays 1 and 2 ON. As soon as the heater H has received the power from relay 2, the power starts to heat up the heater H. Charge current flows into capacitor C through resistor RA to cause the terminal voltage of capacitor C to gradually rise. Chargeable voltage is fed so that it can exactly match the characteristics of temperature rising in the oven chamber. The zener voltage in the zener diode ZD is fed so that it becomes close to the input threshold voltage of the logic circuit 3. After completing the desired cooking operation, the oven is then laid inoperative for a while, and then when the start switch SW is again activated, as was done in the first round, relays 1 and 2 again turn ON so that the power can be supplied to the heater H for heating. See the difference from the first-round operation. In the second round, capacitor C is not charged from zero volts, but rather, the charge starts from a specific voltage level exactly matching the internal temperature of the oven chamber. As a result, the preheating period for the second round becomes shorter than that of the first round until the terminal voltage of capacitor C reaches the input threshold voltage of the logic circuit 3. The preheating period during the second round indicates a monotonous increase that corresponds to the laid-off period of the oven unit.
FIG. 3 shows a conventional preheating timer, in which the terminal voltage of capacitor C is held at a specific level close to that of temperature in the oven chamber, and thus, in principle, it functions as a preheating timer. However, this unit still has a certain disadvantage that is described below. First, since a relay is used in the preheating timer circuit, a large external size is required, thus increasing cost. Second, since a certain variation is present both in the zener voltage of zener diode ZD and the input threshold voltage of the logic circuit 3, a certain difference may be generated in the timer operation. A typical example of incorrect operation of a timer is described below. For example, it is assumed that the zener diode ZD has 7 V of the zener voltage, whereas the logic circuit has 6.8 V of the input threshold voltage, being slightly lower than the other. In this case, when the oven is reactivated for the second-round cooking operation from a certain off period after completing the first-round cooking, the preheating timer may not be operative even when temperature in the oven chamber is lower than the threshold temperature Tth. As described earlier, after completing the first-round of cooking, the voltage in capacitor C is immediately brought down to 7 V of the zener voltage by relay 2, and then it is gradually discharged. On the other hand, if the operator starts the second-round cooking within such a period shorter than the discharge time-constant, since the voltage in capacitor still remains higher than 6.8 V of the input threshold voltage of the logic circuit 3, the voltage output from the logic circuit 3 goes Low. This reduces the operating time of the preheating timer to zero, thus causing the preheating effect to become insufficient. On the other hand, take for example such a case in which the zener voltage remains at 7 V, whereas the input threshold voltage of the logic circuit 3 is 7.2 V, being slightly higher than the other. In this case, since a certain period is additionally needed until capacitor C can be charged up to the input threshold voltage level which is slightly higher than the zener voltage, the preheating timer needs to operate for such a period longer than that is normally required, thus resulting in the excessive preheating effect. Generally, either the zener voltage of zener diode ZD or the input threshold voltage of the logic circuit 3 unavoidably generates a difference of +/-0.1 V through +/-0.5 V, and as a result, the timer cannot completely eliminate such a basic difference.